The present invention relates to a liquid crystal display (LCD), and more particularly, to a static electricity breakdown preventing device which prevents an insulating layer of a thin film transistor (TFT) of the LCD from being destroyed due to static electricity.
FIG. 1 is a schematic block and equivalent circuit diagram of a conventional TFT-LCD. Referring to FIG. 1, a plurality of gate lines 4 and signal lines 5 are arranged perpendicular to each other. A pixel TFT 2 and a liquid crystal capacitor 3 are connected to each gate line 4 and signal line 5 at the intersection where gate line 4 and signal line 5 meet. The plurality of signal lines 5 and gate lines 4 are coupled to a data drive circuit 6 and gate drive circuit 7 formed around the pixel portion, respectively.
In the TFT-LCD constructed as above, electrostatic discharge can occur when a high static electric potential is coupled across signal line 5 and gate line 4. The discharge can destroy the insulating layer of the TFT during the LCD fabrication process. This may cause the device to fail. For the purpose of preventing electrostatic discharge, as shown in FIG. 1, a common line 8 is formed on the periphery of signal line 5 and gate line 4, and static electricity preventing circuit 1 is connected to each signal line and common line, and to each gate line and common line. By doing so, when the static electricity is induced, currents flow through static electricity preventing circuit 1 so that the same potential appears across signal line 5 and gate line 4, thereby preventing insulator breakdown.
FIG. 2 shows a conventional static electricity prevention circuit, consisting of a two-terminal thin film transistor circuit. The circuit includes the gate and two transistors 1A and 1B having their gates and drains respectively tied together.
The conventional static electricity prevention circuit has a current-voltage characteristic as shown in FIG. 3 when a voltage is applied across terminals (a) and (b). This static electricity preventing device operates in such a manner that at a relatively low voltage within a TFT-LCD operation voltage range, a small amount of current flows creating little signal disturbance. On the other hand, if a high voltage is generated, a significant amount of current flows.
However, the conventional static electricity preventing circuit becomes inoperable if one of the two transistors is defective. TFT's typically become defective due to poor gate insulation which causes a high leakage current between the gate and source or drain. In the conventional static electricity preventing circuit, in the event that the gate insulating layer fails, the gate and signal lines are shorted and, consequently, the pixel located at that gate and signal line becomes defective. In addition, the entire pixel and row in which the defective pixel is located do not operate properly.
Furthermore, for a signal level above its threshold voltage, current flows even if the signal voltage is low. This is because the conventional static electricity preventing circuit has a structure in which the gate and drain are connected to each other. Accordingly, a driving circuit having a very high current supply capacity is required. Otherwise, the signal voltage is distorted.